Tunable cavity resonator



April 17, 1956 R. J. BONDLEY TUNABLE CAVITY RESONATOR Filed Aug. 1l. 1952 inventor: Ralph J. Bonde,

by H Attorneg.

Unid States Patent My inventionl relatesto cavity resonators andhasy particular reference to resonator tuning means. y j The resonant circuits for most-.forms o microwave or ultra-'high frequency equipmentvare cavity'resonators whose resonant frequencies aref determinethbyfthe` cavity geometry and thel mode of..oscillation.V .Forganyparticular mode of oscillationhthe freguencyemaybe changed by altering Athe cavity fs-ize, such. alsl,.by. moving; ione, or moreof .the walls of thecavity resonator dnmelatively low power-devices electricall continuity ofthe-movable element is usual-lyrvrnaintain'ed by `meanso sliding plungers or contact iingers.- Such slid-ing contacts,.howeve r, ,are not generally suited .for use. either-at highipower or' in vacuum since the sliding met'ai surfaces-.may .be Welded to each other. In addition, a poorcontacnbetwqenthe sliding members Willgenera-liy lower the Q orsharpness of ICC parta-l' views ofi-.the apparatus of Fig.- 1 illustrating. different positions of the movable cavityv resonator' wall; Fig.l 31is'asectional.view of. theapparatus of Fig.

Fig. '41isfanz end: viewof. a rectangular resonatorA embodyying my invention; and Fig; l is: asectional-side View of another-cavity'resonator embodying myinvention;

w Referringl now to Fig- 1 a-po1ftionI of a resonator surrounding a resonantrspace or cavity 1 is. illustrated in v whichfa-.hollowf outer conductor assembly 2 is terminated one-end by an' inner conductive cylinder vor piston-f3 which'viscoaxial with the outer conductor but spaced therefrom.. The facey of the piston defines the resonator wall and the cylindrical surface of the piston serves. as afgnide asvexplained in the following paragraphs.. The back side or end of the concentric conductor assembly beyond. the piston 3 is closed by a transverse disk 4I which isyrsecured tothe end ofthe hollow outer conductor f assembly 2. A center rod 5 which slides through a centrai bore in the end disk 4 has `its inner ends securedto the/.piston 3,* its external end being available for adjustment of.' the axial position of the inner cylinder. lWhile tuning of the resonator.4 .-In some types of high power vacuum equipment theyresonatorsize' is sometimescom trolled Without using. slidingcontacts:'bfysideformingwthe resonator walls, o'rlby employing bellows,.arrangements, but usually with the accompanyingv d-isadvantagesof lim.- ited tuning range,l low mechanical dura-bility, or undesired induetance. y f ...nia-i. f1

It is therefore. anobjectof my inveniionoprovidea simple andy improved means ttor changingfthe .resonant frequency of a cavity.I resonator by changing jthecavity size-.orgeometryw- It is another object of-fmy` invention toprovideconductive continuityv between relatively 'movable walls or wall portionsf'of: acavityresonator without employing sliding contacts. r 1 5' l' It is a further object of mylinvention means for tuning` a cavity resonator vwhich Ahas relatively long life at either high power or-invacuumgf ;-jq z .l According to my invention, ina cavityffresonatorwhich is tuned by moving one ofits walls-ora portion thereof, the opposite ends of a :plurality of thin, flexiblemetal straps are rmly xed to the opposing relatively frnovable resonator'surfaces.ffhestraps are-narrow. and are long in relation `to thel spacing ,between the .interconnected members softhat acentral portion 'of each strapcxtends/.in a given direction between the conductive,surfaces` as a single loop, the curved portion of Whch'shifts orrolls with the(displacement of. one of the resonator. walls or portion thereof. The straps are closelyspaced so that the Q or bandwith of the tunable cavityfresonators .is-main electricalcontact may be made between the inner and outer conductors if the end disk 3- and .adjusting rods 4 are made of` metal, .no current flows and the resonator tuninguis not affected. the end assembly being utilized only '5to adjust the position of the piston wall 3- and mainf tain its concentricity with respect to the outer conductor .-2. l According to my invention the actual conductive coupli'ngbetween the inner end wall or. piston 3 and the outer kconductor v'assembly 2 is provided byfa plurality oftin fiexibvleconductive straps 6. Eachfstrap is lfasten'edgat onejend to the outer conductor assembly 2 and at 4the otherl endv to the. piston 3 betweenthe opposing cylindrical surfaces thereof and is substantially longer b'lyeach strap has a U-shape with the bend or loop eX- 'tending axially'toward'the back side of the cavity. The

parallel sides ofthe U-shaped straps are biased by the lresilience of the straps against the OPPOSing concentric conductor surfaces, the relative lengths of the sides varyingwith the piston portions. i l To" 'provide a iirrn conductive support the-ends of each strap are preferably both clamped and solderedin place.

end portion 7 of each end of the straps 6 is accordingly bent up 90 degrees and inserted ina slot in the conductive 'member into'which it is to be soldered. Oneend portion 7 of `eachstrap is positioned lin acircu'rnferenv tial Ygroove 8 in the piston 3 around its outer periphery nearits end surface defining the resonator end wall and Vsoldered in4 place. For convenience in soldering, the sides v' of groove 8 may be peened together to `clamp thestrap tained essentiallythe saine asaxedrirequency-.solid wall A' groovenforthe outer ends fof the straps is conveniently formed 'by vdividing the outer conductor assembly 2 into two vlengthwise sections l9 and 10-with` the ysection 9'deiining the front or current carrying portion of the cavity resonator outer wall, and the section 10 de'ning the 'back or inactive portion ofthe resonator assembly. The outer ends 7 of the 'straps are interleaved between the lzvutted` endsof the conductor portions 9 and'1'0 and preferably soldered in place. One suitable way in which tins maybereadily accomplished is shown in Fig. l,

wherey the relative diameters of the ends of the conductor sections '9 and 10 are selected so that section 10 fits into the section 9.y The strap ends are preferably positioned against an internal shoulder 11 of section 9 before assemblygof the conductor sections 9'and l10 and soldered in place.. lThe external portion 12y of the conductor joint may also besoldered or brazed to increase the strength of the assembly.

The limits of the tuning range available through vary- 3 ingthe axial position of the piston 3 is indicated by Figs. 2n and 2b, the bend or loop of the U-shape strap rolling from near one end of the strap to the other end as the piston is displaced in either direction from the central position of Fig. l. Fig. 2a represents the low frequency tuning limit as the piston is moved towards the back end of the resonator to increase the cavity size. lt will be noted that in this case the inductance of the straps 6 is at a minimum since the area between the cylindrical surfaces of the conductor 2 and piston .3 enclosed by the strap is very small. Fig. 2b shows the high frequency limit as the piston is moved into the cavity to decrease its size. In view of the enlarged area between the strap and the conductor 9, the inductance and hence the voltage therebetween may be large enough to cause arcing in high power installations. Recognizing this limitation, the spacing between the facing conductive surfaces may be decreased to thereby decrease the enclosed area within the loop, and the strap length may also v be designed to obtain the desired frequency range without utilizing the full tuning range mechanically possible. p

ln order to preserve the rolling action of the straps `that occurs when the frequency of the resonator is changed,

the facing cylindrical portions of the sides of the piston 3 bending of the straps is thus avoided, as is'essential for long life of the tuning assembly. lt should be noted that while the sides of each U-shaped strap lie against the adjacent conductive surfaces no current is conducted therebetween as the only current carried by the straps 6 is along their inner surfaces, i. e., the surfaces facing the interior of the resonator. There is, therefore, no sliding action and no tendency for the straps to become welded in place due to currents conducted by them.

The number and spacing of the resilient straps 6 may be seen in Fig. 3 which represents a cross section view of the resonator of Fig. l. Each strap is` suflciently narrow so that its width as compared with the degree of arc of the curved conductor portions between which it is fastened is so small as to permit flexing of the strap without binding. The average spacing between adjacent straps is su'ieiently small so that the straps vin effect produce a solid conductive wall for the transmission of radial currents between the inner and outer conductors. This condition is met when the space between adjacent straps is small as compared to a quarter wavelength at the resonator frequency. The Q of such a resonatorlis essentially the same as that of a fixed frequency solid wall resonator.

To maintain both long life of the tuning system and ease of tuning, the straps 6 must be both very thin and very flexible. The strap metal accordingly is chosen for its mechanical characteristics. Stainless steel has been found very desirable in this respect, chrome iron also being satisfactory. The surface of each strap facing the interior of thc resonator is coated with a thin layer of higlrconductivity metal, silver or copper being satisfactory. Since at high frequencies the depth of. penetration of the current is very slight the coating layer is so thin that it does not affect the mechanical properties of the main body of the strap. ll have found a suitable coinbination for strap construction to be a strap of stainless steel approximately .003 inch thick coated with approxi'- mately .0005 inch of copper. Such a strap, despite its heating during the soldering process, will withstand many y the conductorhaving top and bottom walls 14 which are longer thany the side walls 1,5. The end wall or piston 16 is also rectangular, each edge of the movable wall 16 being spaced from the facing surface of the outer wall and connected thereto by a exible strap 17 similarly connected as are the straps 6 of Fig. l. However, since the facing surfaces of the relatively movable conductor walls are rectilinear the width of the strap is coextensive with the length of ytherectilinear surface. Accordingly, only four straps 17 need be employed as compared with the relativelylargenumber of strapsnecessary between the curved surfaces of- Fig. 3. For a simple and common mode of Aoscillation corresponding to the TEoi mode in a rectangular waveguide, the current ow across the end wall 16 is only in a vertical direction between the longer sides 14 of the rectangular cross section. Accordingly, for such a simple mode, only the top and bottom straps 17 are necessary and the side straps may be eliminated.

Another example of a structure embodying my invention is illustrated in Fig. 5, where a resonator 18 surrounds aportion of the length of the path 19 of an electronl stream of`a klystron or other electron discharge device.` InV this case it is desirable to tune the cavity resonator. 18 overfthe desired range while maintaining the resonator vacuum-tight. The cavity resonator 18 is essentially a hollow metal cylinder having annular closures atieither'end whose central openings permit passageof-thev electron streamtherethrough. Accordingly, as'shownin Fig. 5, hollow tubular envelope portions 20 and 21 are respectively hermetically secured to the annular side walls of the resonator. With this type of cavity resonator an entire side wall is not readily movable for tuning'the resonator, and instead, a part of one Side wall is movable to change the capacitance between the opposite sides. Accordingly, a pair of coaxial hollow cylindrical sections 22 and 23 coaxial with the electron stream exten'dffrom opposite side walls toward each other, the'spacingfbetween their facing ends determining the capacitance between them and setting the resonator frcquency. The position of one of the cylindrical sections 23 is axially adjustable to tune the resonator over the desired range, the tuning cylinder 23 being spaced between concentric inner and outer portions 24 and 25 of one ofthe end walls of the resonator.

In accordance with my invention the conductive coupling from each of the wall portions 24 and 25 to the tuning member 23 is provided by two concentric arrays of exible straps 26and 27 similarto those described in connection with the embodiment illustrated in Fig. l.

The innercircular array of straps 26 is secured with the opposite strap ends conductively fastened between the internal surface of the cylindrical tuning member 23 and thefacingsurface Vof the inner wall portion 24. The outerarray. of straps27 is similarly connectedwith one end of each' strap fastened to the outer surface of the tuning member 23 and theother end to the inner surface of the outer wall portion 25.

As` previously described, each of the exible straps 26 and 27 is'relatively narrow and is positioned to detine a U-shaped member opening into the resonator cavity, the opposite sides of each U-shaped strap changing in relative length as the member 23 is axially moved. The single bend or loop of each strap thus rolls along the lengthof the strap, the strap sides lying against the concentric axiallyy extending surfaces of the moving and stationary members. As also previously described, the ends of each strap may be suitably secured by providing radial grooves in the conductors, thetedges of the grooves beingtpeened'over after the strap ends are inserted to clamp them while they are being soldered. In order that the assembly may withstand the temperatures required for exhaust oft-the discharge device of which the resonator is apart a high temperature solder is employed.

While Vthe. inner and outer rows of straps 26 and 27 are relatively/closely spaced, similarly as are those of Fig. 3, to provide an effective electrical termination of the cavity and maintain a high Q, additional means are required for a hermetic seal. For this purpose a suitable metallic double bellows 28 is employed, the bellows having concentric inner and outer members hermetically sccured at one end to the inner and outer end wall portions 24 and 25. The other ends of the belows members are hermetically secured to the end of the tuning cylinder 23 extending outside the resonator. Suitable tuning rods 29 are connected to the external end of the tuning cylinder 23 inorder that the axial position of the tuning member may be adjusted. A housing member 30 extending between the outer wall portion 25 and the discharge device envelope portion 21. is bored to permit passage of the tuning rods 29 therethrough, thus providing a bearing surfacefor maintaining the concentricity of the member 23 and avoiding undue stress on the flexible straps.

The high frequency currents, of course, are all contained within the cavity resonator and the housingmember 30 plays no part in either the vacuum envelope or the conductive circuit of the resonator. The high frequency currents of the resonator travel on the inner surfaces of the straps 26 and 27 and do not iiow through the bellows 28 where the relatively high inductance would be undesirable.

While the present invention has been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as ycome within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is: f

1. An ultra-high frequency apparatus comprising a hollow outer conductor and an inner conductive member coaxial therewith and spaced therefrom, and conductive coupling means therebetween comprising a plurality of flexible straps each having one end secured to the outer conductor and the other end secured to the inner conductor, each of said straps being substantially longer than the spacing between said inner and outer conductors with the central portion thereof exed in a loop extending in an axial direction to permit axial movement of said conductors with respect to each other. y

2. A cavity resonator comprising a hollow outer conductor and an inner conductive member coaxial therewith and spaced therefrom, means for changing the resonator configuration comprising means external of the resonator for maintaining the concentricity of said members when they are moved axially with respect to each other, and conductive means for connecting the opposing surfaces of said outer conductor and said inner conductor at one end of the cavity resonator said conductive means comprising a plurality of liexible metal straps with each strap having its opposite ends secured to the opposing conductor surfaces, said straps being substantially longer than the spacing between said inner and outer conductors with the central portion thereof liexed in a single axially extending loop.

3. An ultra-high frequency resonator comprising an inner and an outer concentric conductor section, means for providing relative axial movement between said sections, and conductive coupling means between said sections comprising a plurality of resilient straps having their opposite ends secured to the facing surfaces of the inner and outer conductor sections, each of said straps being substantially longer than the spacing between said inner and outer conductors with the central portion thereof flexed inl a single loop extending in an axial direction away from the interior of the resonator, the respective sides of the loop so formed lying against said facing conductor surfaces.

4. An ultra-high frequency resonator comprising an inner and an outer concentric conductor section, means for providing relative axial movement between said sections, and conductive coupling means between said sections comprising a plurality of resilient metal straps having their opposite ends fixed to the facing surfaces of the inner and outer conductor sections, each of said straps being substantially longer ythan the spacing between said inner and outer conductors with the central portion thereof iiexed in a single loop extending in an axial direction away from the interior of the resonator and the respective sides of the loop biased against said facing conductor surfaces, the spacing between adjacent straps being relatively smallwith respect to one quarter wavelength at the resonator frequency.

5. An ultra-high frequency resonator comprising an inner and an outer concentric conductor section, means for providing relative axial movement between said sections, and conductive coupling means between said sections comprising a plurality of resilient straps each being substantially longer than the spacing between said inner and outer conductors and having its opposite ends secured to the facing surfaces of the inner and outer conductor sections with the central portion thereof iiexed in a single loop extending in an axial direction, each loop having a highly `conductive concave surface facing the interior of the resonator.

6. An ultra-high frequency resonator comprising an inner and an outer concentric conductor section, means for providing relative axial movement between said sections, and conductive coupling means between said sections comprising a plurality of resilient straps having a highly conductive coating thereon, each of said straps being substantially longer than the spacing between said inner and outer conductors and having its opposite ends fixedto the facing surfaces of the inner and outer conductor sections with the central portion thereof liexed in a single loop extending in an axial direction away from the interior of the resonator with the conductive coating facing the resonator interior, the spacing between adjacent straps being small with respect to one quarter wavelength at the resonator frequency.

7. An ultra-high frequency resonator comprlsing an inner and outer concentric conductor section having current facing surfaces, means for providing relative axial movement between said sections, conductive coupling means connected between said sections comprising a plurality of narrow resilient straps having a highly conductive coating thereon facing the interior of the resonator, each of said straps being substantially longer than the spacing between said inner and outer conductorsand having its opposite end portions lying against sa1d facing surfaces with the central portion thereof flexed in a single loop extending in an axial direction away from the interior of the cavity.

References Cited in the le of this patent 

